The Sith and the Jedi are just minor parts of one same Order of Force, its crowd.
There is no real opposition between them, just usage as a hammer and an anvil to put the galaxy plebs in between.
Sometimes they migrate to the opposite faction. Say, Palpatin and Anakin were originally listed at the Light/Republic/Jedi side, but later were moved to the Dark/Imperial/Sith side.
The Rebels have huge space fleets which could not be built and supported secretly, without the industry, controlled by the empire.
Thus, somebody tells Empire to look in another direction.
They are Forced to use primitive lightsabres (darksabres for Sith?) as a sign of their belonging to the Order, which gives the magic crystals for both.
As they usually migrate from Light to Dark, we can treat it as a promotion.
1. The Dark/Sith are more individual, unique, have more official power, don't need to hide.
2. The Light/Jedi are closer to the simple people, have more understandable, populist, socially appropriate party platform.
So, while the Imperial Troopers are a way to recruit an expendable dumb cannon fodder, the Rebels are a way to recruit more intellectual and active individuals, to let them pass though a real probation without any guarantees, become an autonomous experienced actor, disappoint in Jedi and become Sith, get more power, and be used for more serious tasks than whipping of peasants